refactor(ui): unify list-linked-repo predicate into RepoLinkage

ListsIslandViewModel and SettingsModalViewModel each re-implemented "does
this list have a linked WorkingDir" with different whitespace handling.
RepoLinkage.IsLinked/IsLinkedInDb is now the single definition; both
callers derive from it, closing the whitespace-only WorkingDir gap where
the Settings modal disagreed with the ListsIsland banner.

src/ClaudeDo.Ui/Services/RepoLinkage.cs

@@ -0,0 +1,19 @@
+using System;
+using System.Linq.Expressions;
+using ClaudeDo.Data.Models;
+
+namespace ClaudeDo.Ui.Services;
+
+// Single definition of "does this list have a linked repo" — a WorkingDir that is set to
+// something other than blank/whitespace. ListsIslandViewModel and SettingsModalViewModel each
+// answer this over a different scope (UserLists in memory vs. all Lists rows in the DB) and need
+// it in different shapes, but both must agree on what "linked" means.
+public static class RepoLinkage
+{
+    public static bool IsLinked(string? workingDir) => !string.IsNullOrWhiteSpace(workingDir);
+
+    // EF Core can't translate string.IsNullOrWhiteSpace against Sqlite, so the DB-side check is
+    // spelled out via Trim() instead — same semantics as IsLinked above.
+    public static readonly Expression<Func<ListEntity, bool>> IsLinkedInDb =
+        l => l.WorkingDir != null && l.WorkingDir.Trim() != "";
+}